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Teaching AS Biology Practical SkillsAppendix 2

University of Cambridge International Examinations 2006

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Practical 11 - Investigating the role of carbon diox ide in living organisms.

This practical focuses on manipulation and observations, recording and presentingdata, analysis, drawing conclusions and evaluating methods. The practical alsodevelops skills of using material in new and unfamiliar situations.

Intended learning outcomes

By the end of this practical you should be able to:

Identify dependent and independent variables

Make a hypothesis and express this in words

Experience relevant methods, analysis and conclusion.

Describe and explain the relationship between different living organisms andthe production of carbon dioxide

Evaluate procedures

Safety Information

You should wear eye protection throughout this practical.

Ethanol is highly flammable. There should be no flames in the same room.

Bicarbonate indicator solution is flammable.

Background information

Carbon dioxide is a gas found in the air at 0.04%

Carbon dioxide dissolves in water to form carbonic acid thus reducing thepH

When bicarbonate indicator solution is equilibriated with air it turnsred/orange

Bicarbonate indicator changes colour in different levels of pH

You will remember from biology learnt in earlier courses that plants bothrespire and photosynthesise.

Respiration glucose + oxygen carbon dioxide + water + energy

Photosynthesis carbon dioxide + water glucose + oxygen

The point at which the carbon dioxide released by plants from respiration,equals the carbon dioxide absorbed by plants for photosynthesis is calledthe plants compensation point.

You will investigate the effect of different living organisms on bicarbonate indicatorand use this information to devise an experiment to determine the compensationpoint in plants.

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Read the information above

Identify and write down the dependent and independent variables

Write down what you think will happen (do not worry about what the colourthe indicator will be you will discover that by doing the experiment)

Identify any variables that should be controlled and outline how this shouldbe done

Write down a hypothesis to explain what will happen to the colour of thebicarbonate indicator when a plant is at its compensation point.

Method

Preparations and making observations

1. Rinse out three large test-tubes with distilled water and then withbicarbonate indicator solution.

2. Using a syringe or small measuring cylinder place 3 5 cm3of bicarbonateindicator solution into each test-tube.

3. Carefully place a piece of perforated gauze in each test-tube so that it isjust above the indicator solution.

4. Place a rubber bung or cork into the first test-tube.

5. Carefully place three green seedlings onto the gauze in the second test-tube and seal with a rubber bung or cork.

6. Carefully place three fly larvae onto the gauze in the third test-tube andseal with a rubber bung or cork.

7. Place the three test-tubes near a bright light source such as a lamp orwindow

8. Check that the colour of the bicarbonate indicator solution in each test-tube is red/orange at the start of the experiment.

9. Leave the tubes for at least 30 minutes, comparing the colour of eachindicator solution every ten minutes.

10. When the colours look different in all three test-tubes, note the final colourof the indicator in each of the three test-tubes.

green seedlings

larvae

gauze

bicarbonate indicator

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Write-up

Record your results in a clear table.

Explain why one of the test-tubes contained no living material

Explain your findings using your knowledge of respiration andphotosynthesis

Assess the reliability of the results obtained and suggest any modificationsyou could make to improve the experiment

Plan and describe, but do not carry out an experiment using the sametechnique, to determine the compensation point in plants.

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Practical 11 -Lesson Plan

Investigating the role of carbon dioxide in living organisms.

Context

A practical investigation set in the context of 9700 syllabus Gaseous exchange

Key aims of the lesson

This practical is designed to develop the skills of observation, analysis and evaluationand using knowledge gained in a new and different context

Intended learning outcomes

By the end of the practical and the write-up the student should be able to

Experience relevant methods, analysis, conclusions and evaluation.

Describe and explain how an experimental method can be adapted todiscover when a plant is at its compensation point.

Resources required

White board or flipchart and suitable pens or blackboard and chalk

Practical materials specified on the Technical Information Sheet.

Copies of the student worksheets.

Planned activi ties

Timings/minutes

Teacher/ Student Activities

End ofpreviouslesson

Preparation 2 page student worksheet given out for students toread in preparation for the practical lesson. To consider identificationof the variables, formulate a hypothesis and review previous learningon cell membranes

0 - 3 Introductionto the aims, intended outcomes and shape of the lesson teacher led oral presentation

3 - 5 Context review of pH indicators such as litmus and universalindicator and that carbon dioxide dissolves to form an acidic solution.

5 - 8 Introduction to method Teacher briefly outlines method andanswers any student questions on procedure. Teacher emphasisessafety concerns and ethics when handling living material such as flylarvae which must not suffer undue stress.

8 - 40 Carrying out the practical students carry out the practical work..

40 - 50 Obtain results Students enter results into table and clear awayapparatus as soon as they have finished

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50 - 60 Drawing together the threads Teacher led discussion on the skillsthat have been developed as well as discussion on results obtained.Practical write up to be completed in following lesson or as homeworkactivity

Useful information

Discussion / evaluation points should include:

what colour bicarbonate indicator tuned in different situation

the cause of the colour change in the bicarbonate indicator

what other variables could have affect the results and which variables shouldbe controlled

how the procedure could be improved to increase reliability

how the procedure could be modified to determine the compensation point inplants

A numerical value of the compensation point can be determined by using a lightmetre. The light reading should be taken as close to the plant as possible at the timewhen the plant is at its compensation point.

For students unable to obtain accurate data, the following table of results may beused.

no living material seedlings fly larvae

start red/orange red/orange red/orangecolour ofbicarbonate

indicator end red/orange purple yellow

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Practical 11 -Technical information

Investigating the role of carbon dioxide in living organisms.

The apparatus and materials required for this practical are listed below.

The amount of apparatus listed is for one student or one group of students if they areto work in groups.

1. 3 large test-tubes each fitted with a rubber bung or cork

2. gauze or similar to support the specimens in the test-tubes whilst at thesame time allowing the transfer of gases

3. supply of distilled water to rinse each test-tube.

4. 40 cm3 of bicarbonate indicator solution, sufficient to rinse each test-tubeand have sufficient remaining to place 5cm3into each test-tube.

The stock solution of indicator can be prepared by dissolving 0.2g of thymol

blue and 0.1g of cresol red in 20cm3

of ethanol. Also prepare a solution byadding 0.84g of pure sodium bicarbonate to 900 cm3of distilled water. Addthe dyes to this solution and make up to 1 dm3. To prepare the indicator foruse, pipette 25cm3of stock solution into a graduated flask and make up to250 cm3with distilled water.

The solution should be equilibriated with air by aspirating atmospheric airthrough the solution until it is orange/red in colour.

5. 3 germinated seeds such that they have developed green leaves and arephotosynthesising. Cress seeds that have been placed on moist cotton woolin a Petri dish will germinate and develop leaves in only a few days. Timeswill vary depending upon local conditions.

6. 3 large fly larvae that are active and not approaching pupation

7. 10cm3 graduated pipette or measuring cylinder or syringe

Additionally each student will require access to a sink and running water.

Commercial bicarbonate indicator solution is available from most chemicalwholesalers, however it is possible to make up the solution in the laboratory asdescribed above.

Safety Precautions/Risks.

Ethanol = F

Bicarbonate indicator solution = F

A risk assessment should be carried out as a matter of course.

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